In recent years, the types of storage devices installed in storage systems have become diversified because of the widespread use of HDDs (Hard Disk Drives) and SSDs (Solid State Drives). Particularly, regarding the SSDs, their data storage methods include SLC (Single Level Cell), 2-bit MLC (Multi Level Cell), or TLC (Triple Level Cell); and they are characterized in that even if they are the storage devices of the same type, they have different service lifes (the upper limit number of writes) and bit costs.
Generally, the service life of SLC is the longest and becomes shorter in the order of MLC and TLC. Moreover, the bit cost of TLC is the lowest and becomes higher in the order of MLC and SLC. Therefore, with a storage system equipped with SSDs, it is important to locate data appropriately in consideration of the properties of the SSDs.
PTL 1 and PTL 2 disclose techniques that manage or control a storage system equipped with a plurality of storage devices. Specifically, PTL 1 discloses a technique that integrally manage a plurality of storage devices as a pool and provides a host system with virtual volumes.
Furthermore, PTL 2 discloses a technique that enhances response performance to a host system by managing HDDs and SSDs as storage devices of different tiers, automatically relocating data according to access frequency from the host system, and enhancing a data hit rate in a high tier.